Production of steel for electrical sheet material



United States Patent Int. Cl. H01f 1/04; CZlc 7/10, 7/08 US. Cl. 148-111Claims ABSTRACT OF THE DISCLOSURE The process of producing steel fornon-aging silicon steel for continuous-furnace annealed electrical sheetmaterial includes the steps of tapping a steel melt with a carboncontent of 0.03 to 0.05% and an oxygen content of 0.06 to 0.1% unkilledinto a ladle, subjecting the tapped-off melt to vacuum until the carboncontent is reduced to about 0.01%, then adding aluminum in an amountsufficient to deoxidize the melt down to an oxygen content below 0.005%and adding silicon in an amount of 0.5 to 4.5% corresponding to thecontent required in the electrical sheet material, rolling thedeoxidized and silicized melt to sheet material, and continuous-furnaceannealing it for a period of approximately 1 minute at about 900 C. toobtain electrical sheet having a final carbon content of at most 0.005%;and the product thereof.

Our invention relates to a process of producing steel for themanufacture of electrical sheet and strip material.

A right of priority is claimed from a patent application filed inGermany May 19, 1965, Ser. No. B 29,337.

A large proportion of the presently produced electrical sheet and stripmaterial is annealed in continuously operating furnaces for attainingthe required planarity of the final product. The high temperaturesapplicable for the annealing treatment simultaneously improve themagnetic properties. It has been found, however, that the rapid cooling,inevitable when operating with continuous annealing furnaces, has theeffect that the mag netical qualities of the products deteriorate withprolonged periods of time. Such magnetic aging is due to the carboncontent of the finished sheet material. Reliably preventing thisdeficiency requires limiting the ultimate carbon content to a maximum of0.005%. Hence in the production of steel for such sheets, attempts arebeing made to provide for lowest feasible carbon contents, which are atapproximately 0.03%. The still much lower ultimate carbon contentrequired is then achieved by subjecting the sheets to a decarburizingannealing treatment in the continuous furnace. This requires relativelylOng annealing periods, in the order of about 5 minutes, which is highlydetrimental economically.

It is an object of our invention to obviate this disadvantage andthereby greatly reduce the manufacturing cost of electrical sheet orstrip.

Another correlated object of the invention is to devise a process ofeconomically producing non-aging electrical sheet and strip materialwithout departing from the use of the above-rnentioned annealingoperation in a continuous furnace.

Still another object, akin to those mentioned, is to produce electricalsheet material which combines good magnetical properties with betterpunchability than sheets made in the conventional manner.

ice

To achieve these objects, and in accordance with our invention, steelsfor the production of non-aging electro-sheets, annealed in a continuousfurnace and having the conventional Si contents of 0.5 to 4.5% are madeby a process comprising the combination of the following steps:

1) From a molten composition, a steel containing approximately 0.03 to0.05% carbon is tapped unkilled into a ladle, the tapped-off melt havingan oxygen content of about 0.06 to 0.1%.

(2) The unkilled steel is subjected to vacuum treatment until the carboncontent is reduced to about 0.01%. As a rule, this changes the oxygencontent downto about 0.05 to 0.08%.

(3) Thereafter the steel melt is deoxidized by adding aluminum in aquantity sufficient to reduce the oxygen content to less than 0.005%,the required amount of aluminum being 1 to 5 kg. per metric ton of themelt.

The eliminated oxygen causes the formation of deoxidation productsconsisting essentially of aluminum oxide which can be readily removedfrom the steel bath. After removal of the deoxidation products, thedesired quantity of silicon is added. The simultaneous deoxidation withaluminum and silicon would cause the formation of deoxidation productsthat are difficult to segregate and would result in a high ultimateoxygen content in the finished steel.

In the preferred embodiment of the invention, therefore substantiallyall of the required silicon is added subsequent to deoxidation of themelt with aluminum. The particular advantages resulting therefrom willbe further explained hereinbelow.

The deoxidation as well as the addition of silicon may be effectedduring a vacuum treatment so that in a preferred mode of performing themethod of the invention the entire process takes place in one and thesame degassing vessel. This conjointly utilizes the mechanicaladvantages, namely the vigorous intermixing, inherent in the vacuumtreatment.

For deoxidation with aluminum, higher aluminum quantities are added thanis generally customary. This is done not only for the purpose ofobtaining particularly low oxygen contents, but also in view of thedesired compounding of nitrogen. In many cases, therefore, it ispreferable to use aluminum quantities of about 4 kg. aluminum per ton.

It has been found that sheets rolled from steel obtained in accordancewith the invention, require only a small fraction of the annealing timein the continuous furnace than heretofore necessary. For example, sheetswhich heretofore had to be annealed for about five minutes, need beannealed for no more than about one minute if the sheets are made ofsteel produced in accordance With the invention. It is particularlyadvantageous to operate with such a short annealing time, as will befurther explained.

The method of the invetnion affords reliably obtaining the desiredultimate product despite the large operational fluctuations inevitablein the production of lowcarbon steels. For example, if an attempt weremade to produce exclusively by vacuum treatment a steel having a lowcarbon content as well as a low oxygen content, it is necessary thatcarbon and oxygen be just present in the stoichiometric ratio, acondition which is virtually impossible to satisfy in industrialoperations. Furthermore, the attainable ultimate values of the oxygenand carbon contents would remain relatively high. In contrast thereto,the oxygen content in the starting condition of the process according tothe invention is considerably higher than corresponds to thestoichiometric ratio computed for the particular carbon content.Nevertheless, the carbon content is always reliably reduced to lowestvalues, because the high oxygen content promotes the removal of carbon.The subsequent deoxidation with aluminum, utilizing the vigorous mixingmovement in the melt conjointly occurring with a vacuum treatment, hasthe effect of rapidly reducing the oxygen content down to below 0.005%.

Steel made according to the invention may be cast either intoconventional ingot molds or by means of continuous casting equipment.Thereafter the steel is rolled to sheets or hot strips. Particularlyadvantageous is the use of the steel for cold strip production, of whichan example will be described presently in detail.

Hot strip material of about 2 mm. thickness was rolled down tocold-rolled strip of about 0.5 mm. thickness without intermediateannealing. In this example the silicon content was 1.5%, the aluminumcontent 0.11%. For final annealing, the cold-rolled strip was passedthrough a continuous furnace at 900 C. The total annealing time was 2.2minutes of which 1.2 minutes were consumed during the heating-upinterval. Although the actual annealing period of one minute wasextremely short, it sufiiced to completely recrystallize the steel andto reduce the carbon content to 0.004%. The wattage losses at 10,000gauss were 1.86 watt/ kg. The sheets were magnetically resistant toaging.

The total annealing periods depend upon the silicon content of the sheetmaterial. With a low Si content of about 0.5% and a high Si content of4.5%, these annealing periods are approximately 50% longer than thetotal annealing period in the above-described example relating to sheetmaterial with 1.5% Si. It was a surprising discovery that the sheetmaterial made according to the invention also exhibited excellentpunchability, which makes it eminently well suitable as work materialfor punch presses and the like stamping machinery. This may be explainedby the fact that due to the relatively short annealing periods thesilicon oxide film forming on the surface is extremely thin, thusimproving punchability, whereas with the conventional, much longerannealing periods, a correspondingly thicker silicon oxide coating willoccur.

We claim:

1. The process of producing steel for non-aging silicon steel forcontinuous-furnace annealed electrical sheet material, which comprisesthe steps of tapping a steel melt with a carbon content of 0.03 to 0.05%and an oxygen content of 0.06 to 0.1% unkilled into a ladle, the oxygencontent being higher than necessary for maintaining a stoichiometricratio with the carbon content, subjecting the tapped-off melt to vacuumuntil the carbon content is reduced to about 0.01%, then adding aluminumin an amount suflicient to deoxidize the melt down to an oxygen contentbelow 0.005% and adding silicon in an amount of 0.5 to 4.5%corresponding to the content required in the electrical sheet material,rolling the deoxidized and silicized melt to sheet material, andcontinuous-furnace annealing it for a period of approximately 1 minuteat about 900 C. to obtain electrical sheet having a final carbon contentof at most 0.005

2. The process according to claim 1, wherein the silicon is added to themelt subsequent to deoxidation with aluminum.

3. The process according to claim 1, which comprises performing thedeoxidation and the addition of silicon in vacuum.

4. The process according to claim 2, wherein the tapped-off melt issubstantially free of silicon.

5. The process according to claim 4, wherein the added amount of siliconcorresponds to a content of approximately 1.5 in the electrical sheetmaterial.

References Cited UNITED STATES PATENTS 1,277,523 9/1918 Yensen -492,144,200 1/1939 Rohn 75-129 3,021,237 2/1962 Henke 148-111 3,042,5567/1962 Hemmeter 148-111 3,253,909 5/1966 Bishop 75-129 FOREIGN PATENTS33 8,409 11/ 193 0 Great Britain.

OTHER REFERENCES Yensen: American Electrochem. Soc. Transactions, vol.32, pp. -182, 1917.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerU.S. C1. X.-R.

